Traditionally, plastics have been formulated to result in strong, light-weight, durable, and bioresistant polymeric materials. It is the durability and indestructibility that makes plastic the material of choice for many applications. However, these same properties are problems when the plastics enter the waste stream. The recent trend is to create biodegradable most of such plastics being first commercialized in the mid 1980's..sup.1
Among the first `biodegradable` plastics made were blends of non-biodegradable polyolefins with starch which were at best only partially biodegradable..sup.2-4 These plastics are not compatible with waste management infrastructures, such as composting. Moreover, at that time, the appropriate infrastructures capable of dealing with biodegradables did not exist. Instead of composting, these products generally ended up in sanitary landfills.
Landfills, in general, are a poor choice as a repository of plastic and organic waste. Landfills are plastic-lined tombs designed to retard biodegradation by providing little or no moisture with negligible microbial activity. Organic waste, such as lawn and yard waste, paper, and food waste should not be entombed in such landfills to be preserved for posterity. Accordingly, there is a growing trend to divert these materials into composting facilities which allow them to be biodegraded to produce humus or compost. This compost can then be used as a valuable soil additive for new plant growth.
When plastics are designed to be biodegradable, utilizing renewable resources as the major raw material component, the plastics can become part of an ecologically sound mechanism.
Biodegradation of natural materials produces valuable compost as the major product, in addition to water and carbon dioxide. Such carbon dioxide is fixed or neutral and therefore does not contribute to an increase in the greenhouse gases.
The co-pending patent application,.sup.5 describes a method of preparing biodegradable moldable products and films which includes the steps of preparing a biodegradable, hydrophobic, modified starch, and forming a thermoplastic product, comprising the modified starch, and optionally a miscible and biodegradable plasticizer, and/or a compatible filler. A key aspect of the co-pending patent application.sup.5 is that the modified starch polymers described therein are fully biodegradable, as opposed to blends of biodegradable starch compositions with conventional petroleum-based plastics described earlier. Such blend compositions are at best biodisintregable and not fully biodegradable..sup.2-4 In composting, the non-biodegradable components will be persistent in an irreversible build-up of these components in the environment causing reduced productivity and fertility of the soil..sup.6 Even if such `biodegradable` blend compositions, described in the prior art, are partially biodegradable, the resulting compost will have very little value. In fact, these recalcitrant components will be present in the final compost at significantly higher concentration levels than in the original waste mixture..sup.6
Rowell, Schultz and Narayan published an overview on the emerging technologies from materials and chemicals from biomass..sup.7 In the first chapter of that book Narayan discusses the need for environmentally compatible polymers based on renewable resources. In that book is included a discussion of tailor-made cellulose-polystyrene graft polymers which were used as compatibilizers/interfacial agents to prepare cellulosic-polystyrene alloys and wood-plastic alloys (Chapter 5, pp. 57-75). The graft copolymers function as emulsifying agents and provide for a stabilized, fine dispersion of the polystyrene phase in the continuous phase of the cellulosic matrix. U.S. Pat. No. 4,891,404 to Narayan et al..sup.8 discusses a specific nucleophilic displacement reaction used to prepare such graft polymers which are disclosed to be biodegradable thermoplastic copolymers exhibiting a high capacity for stabilized biodegradable blends of polysaccharide and synthetic thermoplastic polymers. The patent discusses the problems relating to the making of cellulose/starch natural biopolymers and the problem of controlling the molecular weights and degree of substitution of such polymers. Earlier papers by Narayan and Stacy et al..sup.9-11 further discuss biodegradable natural/synthetic graft copolymers.
U.S. Pat. No. 5,095,054 to Lay et al.,.sup.12 issued Mar. 10, 1992, discloses the use of water as a plasticizer for starch (referred to as starch "destructurization") in order to make the material processable in for example an extruder. Products derived therefrom, tend to have the problem of rapidly losing water to the environment by evaporation. As a result this type of material tends to become brittle with age. These materials are also highly water sensitive which is undesirable the majority of applications of thermoplastic products.
To address this issue of water sensitivity, the patent also includes various blends of destructurized starch with a variety of synthetic petroleum-based plastics. Such blend compositions, along with earlier starch-filled polyolefins, are at best only biodisintregable and not fully biodegradable..sup.2-4 Similar starch-polyolefin compositions have been reported by the Fertec group..sup.13
U.S. Pat. No. 4,873,270 to Aime et al., 14 issued Oct. 10, 1989, describes blends of polyurethane with for example poly(vinyl chloride) and a carbohydrate such as potato flour. U.S. Pat. Nos. 3,850,862 and 3,850,863 to Clendinning et al.,.sup.15,16 issued Nov. 26, 1974, disclose blends of a naturally biodegradable product, such as tree bark, protein, starch, peat moss, saw dust, etc., with a dialkanoyl polymer,.sup.15 such as poly(alkylene adipate), or with an oxyalkanoyl polymer,.sup.16 such as poly(caprolactone).
The U.S. Pat. No. 4,863,655 to Lacourse et al.,.sup.17 issued Sep. 5, 1989, discloses water-soluble high amylose starch based containing poly(vinyl alcohol). This biodegradable modified starch product intended for loose fill, or "peanut-shell"-type foam packaging applications, for example, contains a hydroxypropoxylated starch having a very low degree of substitution. This type of modified starch is highly hydrophilic and water soluble; the starch contains about 5% by weight propylene oxide corresponding to a theoretical degree of substitution of 0.19. This is a very low degree of substitution compared with the maximum degree of substitution for starch which is 3.0 according to the three available hydroxyl groups on the anhydroglucose repeat unit. The poly(vinyl alcohol) typically used as a blend component further adds to the water-sensitive nature of these materials. In the case of peanut-shell packaging, the water solubility of such starch-based foams is in fact a positive attribute as this allows the material to be disposed of in an environmentally friendly fashion by simply washing them with water down the drain; the material subsequently biodegrades in the sewer system. For other applications, however, which utilize moldable compositions for various packaging applications, fast food cutlery, plates, cups, etc., need for moisture resistance is of ultimate importance.
The prior art on biodegradable materials and blends is restricted to starch-based materials in which the starch component is hydrophilic (water sensitive). No prior art exists on blends containing hydrophobic, thermoplastic modified starches as fully biodegradable products which are readily processable on conventional plastics processing equipment such as extruders, injection molders, etc.
There are a number of patents and publications in the literature relating to modification of starch by esterification and etherification reactions. Most commercial modified starch products have low degree of substitution (DS) levels designed to alter their solution properties for food applications or adhesion to paper. Acetylated starches, for example, have been known for more than 100 years. Starch acetates ranging from about 0.3 to about 1 DS are typified by water solubility..sup.18 Starch esters which are commercially available for consumption, used for example in salad dressings, have a degree of substitution which typically is lower than 0.1 DS. For example, starch derivatives are cleared for food use by the U.S. Food and Drug Administration (FDA) up to a 4% treatment level, which is equivalent to 0.07 DS..sup.19 Highly acetylated starches, historically, were of some interest because of their organic solvent solubility and their thermoplasticity for film and fiber applications analogous to thermoplastic cellulose esters. In spite of this early development, high DS starch esters have not been developed commercially because they could not compete with similar cellulose derivatives in terms of strength and cost..sup.18 Of primary focus were starch triesters, which fell short in strength and impact properties..sup.20,21 Such high-DS starch esters are characterized by their crystalline properties exhibiting clear melt transitions..sup.22 These high-DS starch esters are not biodegradable. Rivard et al. showed that under anaerobic conditions starch esters (as well as other esters of polysaccharide, including cellulose esters) above substitution levels of about DS=1.7 were not biodegradable..sup.23 We have obtained similar results in our laboratory under composting conditions.
In the co-pending patent application U.S. Ser. No. 08/097,550, .sup.5 applicants have designed starch esters with the degree of substitution, prepared by a unique homogeneous base-catalyzed system under anhydrous conditions, that allows us to obtain starch ester compositions having good mechanical properties while maintaining complete biodegradability. This requires such starch ester compositions to have an intermediate degree of substitution, preferably ranging from 0.4 to 2.5 DS, more preferably from 1.0 to 2.0, and most preferably from 1.2 to 1.7 DS. The latter range of compositions have the most preferred balance in mechanical properties, water resistance, processability and the rate of biodegradation. The starch esters prepared by the co-pending patent application,.sup.5 are predominantly amorphous polymers; little or no residual native starch crystallinity remains due to the homogeneous modification process employed. Without being restrictive, the absence of a new crystalline structure for the starch esters produced by this process probably relates to the range of intermediate degrees of substitution to give non-crystalline copolymers. High DS starch triesters approach the structure of a homopolymer having the needed macromolecular chain regularity required for crystallization. In the starch esters the co-pending patent application,.sup.5 on the other hand, the placement of ester groups on the anhydroglucose repeat units probably follows a close to statistically random distribution pattern resulting in irregular macromolecular chains, giving rise to novel amorphous thermoplastics with unique properties.
The concept of blending polymers is not a new one, and a number of combinations of useful blends are known. In fact, polymer blends have become an important subject of scientific investigation in recent years because of their growing commercial importance..sup.24 An example of a miscible blend system includes the polystyrene/poly(phenylene oxide) blends marketed by General Electric under the trade name NORYL which have enhanced dimensional stability compared with polystyrene (PS), while rendering the poly(phenylene oxide) (PPO) component more readily processable..sup.25 Miscibility of the two polymers results in a single glass transition temperature, Tg, for the blend which is composition dependent, as per the Fox-Flory theory (Tg for PS.sup..about. 100.degree. C.; Tg for PPO.sup..about. 210.degree. C.).
In spite of considerable work done on synthetic polymer-polymer blends, lignocellulosic and other natural polymer-based systems like starch and starch derivatives have seen little use for preparing polymer blend and alloy systems..sup.9 The inventors have reported several blends and alloys containing natural polymers for materials applications. Narayan et al. have reported on blends and alloys containing lignocellulosics,.sup.11 starch,.sup.8-10 and cellulose acetate..sup.7,11 Bloembergen et al. have reported on blends of naturally occurring PHB/V..sup.26-31 The terms blends and alloys are often used interchangeably possibly because of the convenience of semantics equating the two concepts. While the term "blend" is a general term for a mixture of two or more polymers, the term "alloy" is generally used to describe a specific type of blend, namely a "compatibilized blend" that offers a unique combination or enhancement of properties..sup.7
In the field of biodegradable materials, the two most critical issues in developing strong and useful blends are: 1) miscibility or compatibility of the polymer blend components, and 2) complete biodegradability (mineralization) of the components. For example, bacterial poly(.beta.-hydroxybutyrate) (PHB) is notable for its properties as a fully biodegradable yet highly crystalline thermoplastic,.sup.32,33 biosynthesized by a variety of bacteria as an intracellular storage material. In the context of usable thermoplastics, the high melting temperature.sup.33 and brittleness.sup.34 limits its use..sup.35 An attractive solution to the limitations of PHB homopolymer is through blending with a second polymer. PHB has been shown to be miscible with poly(ethylene oxide),.sup.36 poly(vinyl acetate),.sup.37 poly(vinylidine fluoride),.sup.38 and poly(vinyl chloride)..sup.39 U.S. Pat. No. 4,393,167 to ICI discloses polymer blends of PHB and PHB/V with chlorinated polyethylene, poly(vinyl chloride), and Polyacrylonitrile..sup.40 All of these petroleum-based plastics introduced as blend components are not biodegradable, and hence such blend compositions are again at best biodisintregable and not fully biodegradable.
Blends of PHB/V with cellulose esters have been reported, however, both environmental and enzymatic assays on the blend films showed a strong inhibiting effect of the cellulose esters on PHB/V degradation..sup.41 As mentioned previously, this is to be expected for such high DS polysaccharides..sup.23
TONE Polymers are poly(.epsilon.-caprolactone) (PCL) resins marketed by Union Carbide. Several U.S. patents disclose their varying degree of compatibility in blends with polyolefins, including polyethylene, polypropylene, polystyrene, polycarbonate and poly(ethylene terephthalate)..sup.42-44 PCL is one of the few synthetic petroleum-based polymers known to be fully biodegradable..sup.45-49 While PCL is a biodegradable polymer, blends of PCL with such conventional non-biodegradable plastics are at best biodisintregable and not fully biodegradable.
Examples of biodegradable blends which meet both requirements of miscibility/compatibility and biodegradability includes blends of the naturally occurring bacterial PHB with synthetic PHB analogs..sup.26-31 Since synthetic PHB of moderate tacticity is biodegradable, .sup.50,51 the blend should also retain this property..sup.31 Another biodegradable blend which has been reported is a blend of PHB/V with PCL..sup.52
The present invention describes a method of preparing a blend system which is compatibilized and contains fully biodegradable components for making moldable products and films. The compatibilized blend system has been designed and engineered by blending biodegradable, hydrophobic, starch esters with biodegradable polyesters. Examples of biodegradable polyesters include poly(caprolactone) (PCL), poly(vinylacetate-co-vinylalcohol) (PVAc/VA), poly(lactic acid) or polylactide (PLA), poly(glycolic acid) or polyglycolide (PGA), and related copolyesters including the various combinations of stereoisomers, bacterial and synthetic poly(.beta.-hydroxybutyrate) (PHB), Poly(.beta.-hydroxybutyrate-co-.beta.-hydroxyvalerate) (PHB/V), and other poly(.beta.-hydroxyalkanoates) (PHA), and aliphatic biodegradable polyesters.
It is desirable to achieve good processability and mechanical properties with the above-mentioned blends. However, there remains a need for such processable products which are also biodegradable. There is a need for developing new starch-based materials which utilize agricultural resources and return those resources to nature in an environmentally sound manner. The present invention provides new polymeric materials which are environmentally compatible.